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Relativistic backward wave oscillator (RBWO) is a promising high-power microwave (HPM) generator. In past decades, many efforts have been devoted to improving its practicality, in which the repetitively operating RBWO has drawn much attention. However, two bottlenecks once limited its development. One is the generation of massive anode plasmas or even damage of the collector, and the other is the RF breakdown of electrodynamic structures. This article describes an RBWO which can operate repetitively with 3-GW microwave power by mitigating these two problems. First, a titanium collector, which shows better performance in resisting electron beam bombardment than the conventional stainless steel collector, is adopted. Meanwhile, water-cooling technology is applied to expedite heat dissipation of the collector. Second, since our investigations demonstrate that an anodic mechanism, not the conventional cathodic mechanism, is the main cause for RF breakdown of electrodynamic structures under strong guiding magnetic field, the titanium material is also used to manufacture the electrodynamic structures of the RBWO in order to increase its power capacity. The advantage of the titanium RBWO is verified in experiments. The application of these techniques, as well as some important surface treatment techniques such as polishing, ultrasonic washing, and vacuum baking, leads to the stable repetitive operation of the RBWO with 3-GW output power and 25-ns microwave duration under a repetition rate of 50 Hz. The successive operation time of the device is up to 20 s. [ABSTRACT FROM AUTHOR]

An oversized mode-selective relativistic backward wave oscillator is proposed. This device operates using the pure TM02 mode in the beam-wave interaction area and transmits a pure TM01 mode in the output waveguide. In the beam-wave interaction area, mode selection is well realized through oversized slow-wave structures with a coaxial inner conductor that provides perfect boundary conditions for the TM02 mode and simultaneously eliminates other modes. A large proportion of the TM02 mode field is eventually exported into the output waveguide in the form of a pure TM01 mode. With the large diameter and high mode purity, the device can be expected to have high power capacity. With a 770 kV, 11.0 kA beam guided by a 3.5 T magnetic field, a 3.2 GW microwave at 9.42 GHz has been obtained in our simulation. The device efficiency is 38%. [ABSTRACT FROM AUTHOR]

The two-way power combiner consisting of two TM01–TE11 serpentine mode converters with a common output has demonstrated high-power capacity and high combination efficiency for two phase-locked relativistic backward wave oscillators (RBWOs). To realize channel power combination for four phase-locked X-band RBWOs, we propose an all circular waveguide four-way power combiner based on the previous two-way power combiner. Four TM01 modes are first combined into two TE11 modes with two TM01–TE11 mode two-way power combiners; then, through two separate 90° bending waveguides and tapered waveguides, the two TE11 modes are combined into one TE11 mode with a TE11–TE11 mode two-way power combiner. Separate calculations show that at 9.22 GHz, the conversion efficiency of the TM01–TE11 mode two-way power combiner is 95.4%, the transmission efficiencies of 90° bending waveguide and tapered waveguides, and the combination efficiency of the TE11–TE11 mode two-way power combiner are close to 100%. Integrated simulations indicate that as four incident ports are fed with TM01 modes with power of 2.5 GW at each port and phase of 0°, 180°, 0°, and 180°, respectively, the total combination efficiency is 94.5%, and the maximum electric field in the four-way power combiner is 774 kV/cm. [ABSTRACT FROM AUTHOR]

Metallic cathodes can be used to generate intense relativistic electron beams for high-power microwave devices. Many previous papers have demonstrated that metallic cathodes usually have a limited lifetime. In this paper, three metallic cathodes, namely, stainless steel cathode, copper cathode, and aluminum cathode, are researched with a repetitive-operation accelerator and an X-band relativistic backward-wave oscillator. The cathode surface morphologies are observed with a scanning electron microscope before and after lifetime test. The experimental results demonstrate that the stainless steel cathode has a very limited lifetime ( <10^3 pulses), the copper cathode has a moderate lifetime ( \sim 10^4 pulses), and the aluminum cathode has a surprisingly long lifetime (at least 10^5 pulses). The different lifetimes of the three cathodes may originate from the different situations of regeneration of microprotrusions on the cathode surface. The large number of regenerated microprotrusions which possess large microscopic field enhancement factors supports the long lifetime of the aluminum cathode. [ABSTRACT FROM PUBLISHER]

A klystron-type relativistic Cerenkov generator, which combines the frequency and phase control properties associated with an X-band overmoded relativistic klystron amplifiers with the high-power and high-efficiency associated with a klystron-like relativistic backward wave oscillator, is presented. After the beam current is modulated by the input cavity and buncher cavity, it achieves a large enough modulation depth to induce the produced microwave frequency of the backward wave output section. Moreover, the lossy materials inserted in the device can isolate the input cavity, buncher cavity, and backward wave output section effectively, and thus prevent the generated microwave from locking the input signal. Particle-in-cell simulations show that for a 1 MW injected microwave with a frequency of 9.30 GHz, the generated microwave power is 2.8 GW, and the beam–wave interaction efficiency is 45%. As the injected power increases from 100 kW to 2 MW, the frequency control width extends from 20 to 125 MHz. [ABSTRACT FROM AUTHOR]

Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator. [ABSTRACT FROM PUBLISHER]